Exception ValueError

gistfile1.py

import kivy
kivy.require('1.0.0')

from kivy.lang import Builder
from kivy.app import App
from kivy.clock import Clock
from kivy.core.image import Image
from kivy.graphics import *
from kivy.uix.widget import Widget
from kivy.uix.label import Label
from kivy.uix.slider import Slider
from kivy.properties import ObjectProperty, NumericProperty
from kivy.config import Config
from kivy.core.text import Label as CoreLabel

class AbstractMixer(Slider):
    
    def __init__(self):
        super(AbstractMixer, self).__init__(min = 0, max = 1)

        self.orientation = 'vertical'
        self.track_meter = 0

    def set_volume(self, value):
        self.value = value

class TrackMixer(AbstractMixer):

    def __init__(self, track):
        super(TrackMixer, self).__init__()
        self.track = track        

class VoidnoisePanel(Widget):
        
    def __init__(self, **kwargs):
        super(VoidnoisePanel, self).__init__(**kwargs)

        self.col_width = Config.getint('graphics', 'width') / 13.0
        self.row_height = Config.getint('graphics', 'height') / 13.0
        self.row_pos = 0
        self.visible_cols = 10
        self.visible_rows = 10
        self.display_name = None
        self.display_label = None
        self.track_max = 64
        self.track_offset = 0
        self.row_offset = 0
        self.font_size = self.col_width / 10

        self.show_master = False
        self.show_return = False

    def track_range(self):
        return range(self.track_max)

    def trigger_update(self, *largs):
        Clock.unschedule(self.update)
        Clock.schedule_once(self.update, -1)

    def update(self, *largs):
        pass

    def layout(self):
        self.x = self.col_width * 3
        self.y = self.row_pos * self.row_height
        self.width = self.col_width * self.visible_cols
        self.height = self.row_height * self.visible_rows
    


class MixerPanel(VoidnoisePanel):
    
    def __init__(self, **kwargs):
        super(MixerPanel, self).__init__(**kwargs)

        self.mixers = []

        self.bind(pos = self.update, size = self.update)
    
    def set_row_offset(self, row):
        pass

    def add_mixer(self, mixer):
        self.mixers.append(mixer)
        self.add_widget(mixer)

    def update(self, *args):
        for mixer in self.mixers:
            self.remove_widget(mixer)

        for index in range(self.visible_cols):
            mixer = self.mixer_for(index)
            if mixer != None:
                mixer.pos = (self.x + index * self.col_width, self.y)
                mixer.size = (self.col_width, self.height)
                self.add_widget(mixer)

    def track_range(self):
        return range(self.track_max)


class VolumePanel(MixerPanel):

    def __init__(self):
        super(VolumePanel, self).__init__()

        for track in self.track_range():
            self.add_mixer(TrackMixer(track))

        self.update()
            
    def mixer_for(self, index):
        return self.mixers[index + self.track_offset]        



class AppStarter(App):
            
    def build(self):
        panel = VolumePanel()
        panel.canvas.clear()
        panel.update()

        return panel
        

if __name__ in ('__main__', '__android__'):
    app = AppStarter()
    app.run()